Positioning with wireless local area networks and WLAN-aided global positioning systems

ABSTRACT

Accurate position capability can be quickly provided using a Wireless Local Area Network (WLAN). When associated with a WLAN, a wireless device can quickly determine its relative and/or coordinate position based on information provided by an access point in the WLAN. Before a wireless device disassociates with the access point, the WLAN can periodically provide time, location, and decoded GPS data to the wireless device. In this manner, the wireless device can significantly reduce the time to acquire the necessary GPS satellite data (i.e. on the order of seconds instead of minutes) to determine its coordinate position.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/309,537 entitled “Positioning With Wireless Local Area Networks AndWLAN-Aided Global Positioning Systems” filed Dec. 1, 2011 which is adivisional of U.S. patent application Ser. No. 12/904,085 entitled“Positioning With Wireless Local Area Networks And WLAN-Aided GlobalPositioning Systems” filed Oct. 13, 2010, now U.S. Pat. No. 8,095,155which is a divisional of U.S. patent application Ser. No. 11/461,335,entitled “Positioning With Wireless Local Area Networks And WLAN-AidedGlobal Positioning Systems” filed Jul. 31, 2006, now U.S. Pat. No.7,899,472 which is a continuation of U.S. patent application Ser. No.10/367,524, entitled “Positioning With Wireless Local Area Networks AndWLAN-Aided Global Positioning Systems” filed Feb. 14, 2003, now U.S.Pat. No. 7,130,646.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to quickly and accurately determining theposition of an end user using a wireless local area network (WLAN).Alternatively, assuming the end user has recently left the WLAN, thepresent invention relates to using the WLAN information to more quicklydetermine the position of the end user using a global positioning system(GPS).

2. Related Art

Location information of a mobile user is increasingly being used in awide range of situations. For example, such location information canhelp a person navigate in an unfamiliar area, thereby allowing thatperson to quickly find a restaurant, shopping center, or some otherdestination. In addition to being useful, location information can alsobe critical in emergency situations. For example, location informationcan be used to reach a person that has dialed 911 from a cellular phone.To ensure that emergency personnel can find that person in need, the FCCmandates that cellular providers report the 911 caller location, knownas enhanced 911.

To comply with this mandate, cellular phones can access a system calledGlobal Positioning System (GPS) to provide location information. GPScurrently includes a nominal operational constellation of 24 satellitesin orbit. The constellation configuration includes six equally spacedorbital planes provided 60 degrees apart and inclined at approximately55 degrees from the equatorial plane. Of importance, this constellationconfiguration allows a receiver to establish contact with multiplesatellites, e.g. between 4 and 8 satellites, from any point on earth.

Although still controlled by the U.S. Department of Defense, GPS isavailable to the general public at no charge and provides high locationaccuracy (e.g. within 100 meters). However, GPS poses two major problemswith respect to wireless devices (e.g. cellular phones). First, GPSreceivers do not work reliably indoors due to either attenuation (e.g.the signal is too weak to be received) or reflection (e.g. the GPSreceiver inadvertently picks up a reflected signal (a phenomenon knownas multipath), and therefore computes the wrong position).

Second, even under ideal conditions, a GPS receiver can take asignificant period of time to compute its position. Specifically,conventional GPS receivers need to decode GPS satellite data before thefirst position can be computed. Because the current data rate for a GPSsignal is only 50 bit per second, the GPS receiver may require up to12.5 minutes to collect the GPS satellite data necessary in a “coldstart” to determine the first position. A cold start means that both theGPS almanac, i.e. the approximate orbital data parameters for thesatellites, and a rough position of the device are not known. Moreover,even in a “warm start” in which the GPS almanac and rough location areknown, but not the GPS ephemeris, i.e. detailed data parameters thatdescribe short sections of the satellite orbits, the GPS receiver maystill require approximately 40 seconds to determine the position of thedevice. This time period may cause users various degrees of adverseresults ranging from mere inconvenience to life-threatening situations.

Other positioning techniques have been proposed, but have not beenacceptable due to their disadvantages. For example, in a positioningtechnique called triangularization, a time difference of arrival (TDOA)is calculated to locate a wireless caller. However, triangularizationrequires at least three reference points (base stations) to get areasonably accurate position, and typically a cellular phone is coveredby only one base station. In another positioning technique provided bySnaptrack, a subsidiary of Qualcomm, a GPS receiver can track satellitesindoors using one or more cell phone networks. However, this techniqueworks only within cellular phone coverage and requires modifications ofcurrent cell phone infrastructure.

Therefore, a need arises for a method of quickly determining an accurateposition of a device using an existing infrastructure.

SUMMARY OF THE INVENTION

A conventional GPS receiver cannot work under certain conditions, suchas when the GPS receiver is inside a building, inside a tunnel, or undera bridge. Under these conditions, the signal from a GPS satellite isblocked, thereby preventing the GPS receiver from determining itsposition. Unfortunately, even when the signal from the GPS satellite isreceived, the decoding of that signal can take a significant amount oftime. This decoding time can be merely annoying, e.g. if a user istrying to navigate from one location to the user's destination, or evenlife threatening, e.g. if the user is in an emergency situation andneeds assistance immediately.

In accordance with one aspect of the invention, a wireless local areanetwork (WLAN) can quickly provide accurate position information to auser via a wireless device (also called a client herein). Specifically,when associated with a WLAN, a client can quickly determine its positionbased on information provided by an access point in the WLAN. Even whena client disassociates with the access point, the WLAN can provide time,location, and decoded GPS data to the client before its disassociation.In this manner, the client can significantly reduce the time to acquirethe necessary GPS satellite data (i.e. on the order of seconds insteadof minutes) to determine its coordinate position.

The method includes identifying a first access point having a firstknown location and a predetermined radio propagation range of r. Theposition of a wireless device, which is capable of associating with theaccess point, is the first known location with an uncertainty of r. Inthis manner, even the relative position of the wireless device is withinan acceptable range of accuracy for emergency service.

To further refine this relative position, the method can further includedetermining a radio signal strength indicator (RSSI) of the accesspoint. The RSSI can be correlated to a distance, which could be storedin a lookup table (LUT) in the wireless device. The stored distancerepresents a more accurate relative distance from the access point tothe wireless device.

To obtain a coordinate position (x0, y0, z0) of the wireless device, thewireless device can receive information from additional access points.Specifically, the wireless device can identify second and third accesspoints, each having known locations. The wireless device can thendetermine second and third RSSIs of the second and the third accesspoints, respectively. The second and third RSSIs can be correlated tosecond and third stored distances. The second stored distance representsa second relative distance from the second access point to the wirelessdevice, whereas the third stored distance represents a third relativedistance from the third access point to the wireless device. At thispoint, the wireless device can compute its coordinate position using thefirst, second, and third known positions and the first, second, andthird relative distances.

For example, if the first known position is (x1, y1, z1) and the firstrelative position is r1, the second known position is (x2, y2, z2) andthe second relative position is r2, and the third known position is (x3,y3, z3) and the third relative position is r3, then computing thecoordinate position includes using a set of equations:x0=(w1*x1)+(w2*x2)+(w3*x3),y0=(w1*y1)+(w2*y2)+(w3*y3), andz0=(w1*z1)+(w2*z2)+(w3*z3),whereinw1=(1/r1)/(1/r1+1/r2+1/r3),w2=(1/r2)/(1/r1+1/r2+1/r3), andw3=(1/r3)/(1/r1+1/r2+1/r3).

In another embodiment of the invention, the use of multiple antennaswithin an access point can also facilitate quickly and accuratelyproviding a coordinate position of the wireless device. In thisembodiment, the wireless device can determine an RSSI associated witheach of the multiple antennas. The signal path of the antenna associatedwith a highest RSSI can be identified. Once again, the highest RSSI canbe correlated to a stored distance, wherein the stored distancerepresents a relative distance from the access point to the wirelessdevice. At this point, the wireless device can use the angles associatedwith the signal path and the relative distance to compute a coordinateposition of the wireless device.

The angles associated with the signal path can include an angle θ, whichis measured taken from an x-axis to the signal path, and an angle Φ,which is measured from the signal path to a path connecting the wirelessdevice and the access point. Assuming the coordinate position of thewireless device is (x4, y4, z4) and the relative position is r4, thenthe wireless device can compute its coordinate position using a set ofequations:x4=x5+(r4*cos(Φ)*cos(θ)),y4=y5+(r4*cos(Φ)*sin(θ)), andz4=z5+(r4*sin(Φ)).

In accordance with one embodiment of the invention, a method ofdetermining a new common position of a first device and a second device(both having wireless networking capability) is provided. In thisembodiment, the first and second devices exchange information in an adhoc communication mode. The shared information includes the lastpositions of the first and second devices as well as the update timesassociated with those last positions. Of importance, access pointspreviously associated with the first and second devices can provide thelast positions and update times.

For example, assume the first client has a last position (x11, y11,z11), which was updated t1 seconds ago, and the second client has a lastposition (x12, y12, z12), which was updated t2 seconds ago. In thiscase, the common position (x, y, z) can be computed using a set ofequations:x=w1*x11+w2*x12y=w1*y11+w2*y12z=w1*z11+w2*z12wherein weighting factors w1 and w2 are defined as:w1=(1/t1)/(1/t1+1/t2)w2=(1/t2)/(1/t1+1/t2).

In accordance with another aspect of the invention, information from aWLAN can facilitate determining a coordinate position of the wirelessdevice using a global positioning system (GPS). Of importance, theinformation from the WLAN can include decoded GPS data, a current time,and a recent position. Note that because the wireless device does notneed to decode a GPS signal itself, significant time (i.e. up to 12.5minutes) is saved. Moreover, using this information, a first GPSsatellite can be quickly identified for searching. This first GPSsatellite advantageously has the highest elevation angle from the totaloperational set of GPS satellites (which minimizes the risk of signalblocking and significantly simplifies subsequent computations associatedwith other GPS satellites).

In one embodiment, to accelerate the searching process, the method caninclude partitioning multiple correlators into correlator pairs (i.e.groups of two) or correlator tracking sets (i.e. groups of three). Thesecorrelators ensure that the time and frequency offsets associated withthe first GPS satellite are accurately determined. Once these offsetsare determined, the time and frequency offsets for additional GPSsatellites can be quickly derived. Position information associated withfour GPS satellites is needed to provide a coordinate position for thewireless device.

Thus, as described above, various methods using a WLAN can be providedfor quickly determining an accurate position of a device having wirelessnetworking capability. In general, the methods include identifying afixed position of one or more access points accessible by the device andcomputing the position of the device using short radio wave propagationbetween the access point(s) and the device.

Advantageously, this computed position can be used to provide emergencyservice for cell phones. Additionally, the WLAN position technique canadvantageously augment the navigation tools of any type of wirelessdevice, e.g. PDAs, cell phones, and notebooks. For example, the wirelessdevice can provide local maps and points of interest for later GPSnavigation with WLAN aided information. These navigation tools could beused from any location, e.g. home, work, airports, restaurants, hotels,etc.

The WLAN position technique can also be used in conjunction withlocation based applications. Specifically, a client can download alocation based application and its related database from the WLAN andthen move to a new location with or without WLAN connections. Forexample, a client could connect to an access point in a coffee shop (orsome other LAN “hotspot”) in Tokyo. At this point, an application servercould quickly determine the client's location through the access pointand then feed local maps along with other local points of interest (POI)to the client through the access point. Using this information, theclient could then use the WLAN position technique or the WLAN aided GPStechnique to navigate to a new location.

A WLAN system for facilitating providing a GPS position of a client isalso provided. The system can include analog means for associating withthe client, an analog to digital conversion means, and digital means forproviding WLAN baseband operation as well as GPS baseband operation. Ofimportance, the analog means can include a frequency synthesizer thatreceives one of a first signal associated with WLAN baseband operationand a second signal associated with GPS baseband operation. The WLANsystem can further include means for providing decoded GPS data to theclient preceding disassociation of the client from the WLAN.

Advantageously, the position techniques using WLAN and the WLAN aidedGPS are complementary. Specifically, if the client is operating within aWLAN, then the client can quickly compute its relative and/or acoordinate position based on information from an access point. At thispoint, no GPS tracking is necessary because an acceptably accuratelocation of the client can be provided solely via the WLAN. When theclient disassociates with the WLAN, the WLAN can provide the client withdecoded GPS data, time, most recent position information, therebyensuring that the client can significantly reduce its searching time forthe necessary GPS satellites in order to compute its coordinateposition.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates how a relative position of a client in a wirelessnetwork, i.e. a WLAN, can be determined using the known position of anaccess point.

FIG. 2 illustrates how an accurate position of a client in a WLAN can becomputed using the known positions of and wireless connection distancesfrom multiple access points.

FIG. 3A illustrates that when multiple antennas are used by an accesspoint, the client can use the angle of the signal path providing thestrongest signal to further refine its position.

FIG. 3B illustrates two clients that can determine a common position inan ad hoc communication mode.

FIG. 4A illustrates a simplified diagram in which one correlator is usedto provide locking of the GPS satellite.

FIG. 4B illustrates a correlator search pair that can be used in caseswhere the carriers are separated.

FIG. 4C illustrates a correlator tracking set that can be used in caseswhere the early/late versions of the C/A code are tracked.

FIG. 5 illustrates exemplary angles, wireless connection distances, andresulting Dopper effects for two satellites.

FIG. 6 illustrates exemplary components in a WLAN system that canadvantageously be shared between WLAN broadband and GPS broadband.

DETAILED DESCRIPTION OF THE FIGURES

In accordance with one aspect of the invention, accurate positioncapability can be quickly provided using a Wireless Local Area Network(WLAN). As used herein, the term WLAN can include communication governedby the IEEE 802.11 standards (i.e. the 1999 standard as well as theproposed 2003 standard), Bluetooth (an open standard that enablesshort-range wireless links between various mobile units and connectivityto the Internet), HiperLAN (a set of wireless standards, comparable tothe IEEE 802.11 standard, used primarily in Europe), and othertechnologies having relatively short radio propagation range. Inaccordance with another aspect of the invention, position capability canbe provided using a WLAN in combination with GPS. These positioningtechniques and their associated advantages will now be described indetail.

Positioning with Wireless Local Area Networks: Infrastructure Mode

One common mode of operation in wireless communication is theinfrastructure mode. In an infrastructure mode, an access point cancommunicate with a plurality of clients associated with the accesspoint. Clients in this WLAN communicate with each other through theaccess point. Thus, the access point functions as a communication hubbetween its associated clients.

An access point can also serve as a gateway for its clients tocommunicate with clients not associated with the access point. Forexample, a first access point can communicate with a second accesspoint, which in turn can be associated with a plurality of its ownclients. Because a client can be a mobile device whereas each accesspoint has a fixed location and a predetermined range of service, aclient may associate with the first access point for a first time periodand then associate with the second access point for a second timeperiod.

In accordance with one feature of the invention, the fixed location andrange of the access point can be used to provide an accurate position ofany of its clients. In one embodiment, the position of the access pointcan be determined (e.g. using electronic mapping or GPS) and stored inits database during setup. Of importance, when a client associates withthe access point, the client's position is thus known to be within theradio propagation range of the access point.

For example, FIG. 1 illustrates that if the position of an access point101 is (x1, y1, z1) (indicating a location in three dimensional space)and the wireless connection range 103 is r, then the position of aclient 102 associated with access point 101 is (x1, y1, z1) with anuncertainty of r. Logically, the longer the wireless connection range103, the larger the uncertainty will be. Advantageously, becausewireless connection range 103 is typically less than 100 meters, thisaccuracy can satisfy most user applications.

A client can further refine its position using a radio signal strengthindicator (RSSI), which refers to the wideband power provided by theaccess point within channel bandwidth. Generally, a client receives astronger signal from the access point when it is closer to that accesspoint. Thus, an RSSI can be correlated to a particular distance from theaccess point.

For example, client 102 could determine its relative position withrespect to access point 101 by using the RSSI from access point 101. Inthis manner, although the coordinate position of client 102, i.e. (x0,y0, z0), would still be unknown, client 102 could determine its wirelessconnection distance 104, i.e. relative position r1, based on the RSSI ofaccess point 101. In one embodiment, this RSSI correlation can be storedin a lookup table (LUT), which can then be accessed by a client torefine its relative position.

Of importance, if a client can associate with multiple access points,then the client can also advantageously use the RSSI corresponding toeach access point to yet further refine its position. For example, FIG.2 illustrates client 102 being within association range with accesspoints 101, 201, and 203. In accordance with one aspect of theinvention, client 102 can communicate with each access point, identifythe fixed location of each access point, assess the RSSI of each accesspoint, and determine a wireless connection distance (i.e. a relativeposition) to each access point. For example, in FIG. 2, client 102 canidentify that access point 101 has a fixed position of (x1, y1, z1) anddetermine its wireless connection distance 103 (i.e. a relative positionr1) to access point 101. Similarly, client 102 can identify that accesspoint 201 has a fixed position of (x2, y2, z2) and determine itswireless connection distance 202 (i.e. a relative position r2) to accesspoint 101. Finally, client 102 can identify that access point 203 has afixed position of (x3, y3, z3) and determine its wireless connectiondistance 204 (i.e. a relative position r3) to access point 101.

Of importance, the location of client 102 (x0, y0, z0) can be accuratelycomputed based on the locations of and wireless connection distancesfrom access points 101, 201, and 203 with the following formulae:x0=(w1*x1)+(w2*x2)+(w3*x3)y0=(w1*y1)+(w2*y2)+(w3*y3)z0=(w1*z1)+(w2*z2)+(w3*z3)where weighting factors w1, w2, and w3 can be computed from thefollowing formulae:w1=(1/r1)/(1/r1+1/r2+1/r3)w2=(1/r2)/(1/r1+1/r2+1/r3)w3=(1/r3)/(1/r1+1/r2+1/r3)

In accordance with another aspect of the invention, if multiple antennasare used in an access point, the client can also use the angles of thesignal path providing the strongest signal to compute its coordinateposition. Specifically, an access point may switch back and forthbetween multiple antennas to determine which antenna provides the bestsignal strength for a particular client. After the access pointidentifies the best, i.e. the strongest, signal path to the client, theaccess point can transmit the angles associated with that best signalpath to the client, which in turn can use those angles to compute itscoordinate position. Note that if the necessary hardware resources areavailable, a client can also identify the best signal path. Further notethat in one embodiment, the access point can compute the coordinateposition of the station and then transmit that position to the station.

FIG. 3A illustrates a client 302 associated with an access point 301,which includes multiple antennas. After measuring the RSSIs from each ofthese antennas, access point 301 identifies the antenna having the bestsignal path, i.e. best signal path 304, to client 302. At this point,client 302 can use the RSSI associated with best signal path 304 todetermine its wireless connection distance 303, i.e. relative positionr4, to access point 301. Access point 301 can transmit an angle e(theta)(taken from an x-axis to the best signal path 304) and an angle T(phi)(taken from the best signal path 304 to a path, which is measuredby wireless connection distance 303, connecting client 302 and accesspoint 301) to client 302. At this point, the coordinate position (x4,y4, z4) of client 302 can be accurately computed with the followingformulae:x4=x5+(r4*cos(Φ)*cos(θ))y4=y5+(r4*cos(Φ)*sin(θ))z4=z5+(r4*sin(Φ))Positioning with Wireless Local Area Networks: Ad Hoc Mode

In accordance with another aspect of the invention, WLAN positioning canbe used in an ad hoc communication mode. In the ad hoc mode, clientscommunicate directly with each other without using an access point orcommunication hub. Advantageously, a client can share certaininformation with at least one other client to compute a common “new”position. This information can include the clients' “old” (e.g. its mostrecent) positions and associated update times for those old positions.Note that the old positions as well as the update times could beprovided by access points (from which the clients recentlydisassociated) or by downloaded and decoded GPS data (described below infurther detail).

For example, referring to FIG. 3B, assume that client 310 had an oldposition (x11, y11, z11), which was updated ti seconds ago, and client311 had an old position (x12, y12, z12), which was updated t2 secondsago. In this case, their best estimate of their best estimate, commonposition will be:x=w1*x11+w2*x12y=w1*y11+w2*y12z=w1*z11+w2*z12where the weighting factors w are defined as:w1=(1/t1)/(1/t1+1/t2)w2=(1/t2)/(1/t1+1/t2).

It should be noted that the above is just one example formula intendedto weight the solution more toward the “newer” position. It will be lessreliable as the parties' positions age. Thus, if time t1 issignificantly smaller than time t2 (i.e. the position of client 310 wasupdated much more recently), then the weighting factor w1 will be closeto 1 whereas the weighting factor w2 will be close to 0. In this case,client 311 would effectively get its position from client 310.

In an area densely populated by access points, the amount of time spentby clients between access points, thereby requiring the use of an ad hocnetwork for communication, is minimized. Thus, the time updates of theold positions (e.g. t1 and t2) are small, which increases theprobability that the old positions are substantially accurate. Ofimportance, the above formulae can be scaled to any number of clientsassociating in the ad hoc mode.

Positioning with WLAN Aided GPS

In accordance with another aspect of the invention, a WLAN canadvantageously provide some limited, but critical, position informationbefore initiating GPS, thereby dramatically reducing the time needed todetermine the position of the wireless device using GPS. As previouslyindicated, a conventional GPS receiver can take up to 12.5 minutes todecode all GPS data. WLAN assisted GPS can advantageously reduce thistime to between 3-6 seconds or less.

GPS: General Overview

In a conventional cold start, a GPS receiver does not know the time, itsapproximate location, or the GPS almanac. Because the GPS receiver doesnot know which satellite of the 24 satellites to search for first, theGPS receiver initiates a search for an arbitrarily chosen satellite. TheGPS receiver continues the search for one or more satellites until onesatellite is found and the GPS data can be decoded.

Of importance, all GPS satellites are broadcasting at the samefrequency. However, each GPS satellite is broadcasting a coarseacquisition (C/A) code that can uniquely identify it. Specifically, theC/A code is a pseudo random noise (PRN) code, i.e. the code exhibitscharacteristics of random noise. Currently, there are 37 PRN codesequences used in the C/A code. Each GPS satellite broadcasts adifferent code sequence. Thus, each GPS satellite can be identified byits PRN number.

The C/A code includes a sequence of binary values (i.e. zeros and ones)that is 1023 “chips” long (wherein a chip refers to a binary valueassociated with identification rather than data). The GPS satellitebroadcasts the C/A code at 1.023 Mchips/sec. Thus, the C/A code can be(and actually is) repeated every millisecond.

A GPS receiver includes correlators to determine which PRN codes arebeing received and to determine their exact timing. To identify a GPSsatellite, a correlator checks a received signal against all possiblePRN codes. Note that a GPS receiver typically stores a complete set ofpre-computed C/A code chips in memory, thereby facilitating thischecking procedure.

Of importance, even if the GPS receiver uses the right PRN code, it will“match” the received signal only if the PRN code lines up exactly withthe received signal. In one embodiment, to facilitate a quick matchingfunctionality, the GPS receiver includes a C/A code generatorimplemented by a shift register. In this embodiment, the chips areshifted in time by slewing the clock controlling the shift register.Thus, for each C/A code, correlation of the PRN code and the receivedsignal could take from 1 millisecond (best case) up to 1023 milliseconds(worst case). Usually, a late version of the code is compared with anearly version of the code to ensure that the correlation peak istracked.

Even after matching the received signal from the GPS satellite, the GPSreceiver still needs to completely decode the GPS data before theposition can be computed. The GPS data structure is provided in fivesub-frames: sub-frame #1 that includes satellite clock correction data,sub-frame #2 that includes satellite ephemeris data (part I), sub-frame#3 that includes satellite ephemeris data (part II), sub-frame #4 thatincludes other data, and sub-frame #5 that includes the almanac data forall satellites. Currently, GPS receivers take up to 12.5 minutes todecode all five sub-frames.

WLAN Provides a Head Start to GPS

In accordance with one feature of the invention, the decoded GPS datacan be either downloaded from a dedicated server in the WLAN network(note that each access point, e.g. the access points shown in FIGS. 1and 2, are connected to a wired network) or could be stored by theaccess point. In either case, assuming that the downloaded/storeddecoded GPS data is still accurate and the GPS receiver (i.e. theclient) remains substantially in the last updated position, determiningthe position could be reduced by 12.5 minutes. In one embodiment, thedecoded GPS data could be transferred to the GPS receiver periodically.For example, in one embodiment, the decoded GPS data could betransferred every 5-10 minutes. Because the decoded GPS data is smallgiven a typical WLAN bandwidth, this periodic transfer should havenegligible impact to WLAN throughput.

In accordance with another feature of the invention, the GPS receivercan quickly compute the best satellite to search for, i.e. the satellitewith the highest elevation angle (explained in detail below).Specifically, the GPS receiver can compute the satellite with thehighest elevation angle using the GPS almanac data (provided by thedecoded GPS data), the approximate current time (provided by the accesspoint), and the latest position (also provided by the access point).

In accordance with another feature of the invention, the number ofcorrelators can be varied to provide quick and accurate locking of thatGPS satellite. FIG. 4A illustrates a simplified diagram in which onlyone correlator is used. Specifically, a mixer 401 (which performs anexclusive OR logic operation, i.e. a binary multiplication) receives aGPS signal 400 from a satellite as well as a C/A code 403 from a C/Agenerator. An accumulator 402 accumulates mixer output for the completeC/A code period, e.g. 1 millisecond. At this point, accumulator 402 isreset and repeats the process. A single correlator typically takes onthe order of 60 seconds to lock onto the first satellite. (Note thatcomponents of the analog portion of the GPS architecture, such asamplifiers, filters, etc., are not shown for simplicity.) To maintain alock on the satellite, a delay lock loop (DLL) can be used, therebyensuring that the complete GPS data can be downloaded withoutinterruption.

GPS signal 400 is the received GPS signal down-converted to basebandwith an in-phase signal and a quadrature phase signal mixed.Specifically, the GPS signal is a binary phase shift keying (BPSK)modulated signal having two carriers, wherein the carriers have the samefrequency, but differ in phase by 90 degrees (i.e. one quarter of acycle). One signal, called the in-phase (I) signal, is represented by acosine wave, whereas the other signal, called the quadrature (Q) signal,is represented by a sine wave. FIG. 4B illustrates a correlator searchpair that can be used in cases where the carriers are separated. In thiscase, a GPS in-phase signal 405 can be provided to a mixer 406 and a GPSquadrature signal 408 can be provided to a mixer 410. Both mixers 406and 410 receive the locally generated C/A code 409. In this embodiment,the I value is accumulated as is the Q value by accumulators 407 and411, respectively.

By forming a root-sum-square (RSS) of the output C/A_I and C/A_Q values,the GPS receiver can determine whether it has successfully acquired theGPS satellite. Specifically, the GPS receiver is looking for the highestI²+Q² value over all the 1023 different C/A code offset positions. Afterthe highest value (above a programmable threshold), is found, the GPSsatellite is “acquired”. Thus, the carrier tracking loop can be closedbased on the C/A_I and C/A_Q values.

FIG. 4C illustrates a correlator tracking mode that can be used in caseswhere the early/late versions of the code are tracked. In this case, theGPS in-phase signal 420 can be provided to mixers 421 and 425 and theGPS quadrature signal 424 can be provided to a mixer 427. Both mixers425 and 427 receive the locally generated C/A code 429. However, mixer421 receives a early/late version of C/A code 423 (i.e. an in-phaseearly signal minus an in-phase late signal). In this embodiment, the Ivalue, the Q value, and the I early/late (IEL) value are accumulated byaccumulators 426, 428, and 422, respectively. Once again, by forming aroot-sum-square (RSS) of the C/A_I and C/A_Q values, the GPS receivercan determine whether it has successfully acquired the GPS satellite. To“lock” the satellite, known delay lock loop techniques can be used withthe C/A_I and C/A_EL values to close the C/A code tracking loop.

In one embodiment, to shorten the time to acquire (i.e. lock onto) thefirst satellite, multiple correlators in the GPS receiver can be used tosearch on the same satellite in parallel with different time and/orfrequency offsets. Note that both the GPS satellite and the GPS receiveruse oscillators to generate their signals. Typically, the GPS receiveruses a crystal oscillator with a frequency variation of +/−20 ppm (partsper million). Because the GPS satellite signal is located at 1.575 GHz,the frequency offset between the two oscillators could be from −30 kHzto +30 kHz (1.575e9×20e-6=30e3). Thus, locking of the GPS signal caninclude adjusting the GPS receiver oscillator to provide exactly thesame frequency as the GPS signal.

Assuming 24 correlators are implemented in a GPS receiver, thesecorrelators can be partitioned into 12 search pairs (see FIG. 4B) witheach search pair containing an in-phase correlator and aquadrature-phase correlator. In this embodiment, the worst search timewould be 5 seconds (i.e. (60 frequency offset×1 second)/12) and theaverage search time would be 2.5 seconds (i.e. (30 frequency offset×1second)/12). Logically, if more correlators are implemented in the GPSreceiver, the search time could be decreased proportionally. Forexample, if 48 correlators are implemented, then the worse search timeusing search pairs would be 2.5 seconds and the average search timewould 1.25 seconds.

After the first satellite is acquired, the 24 correlators can bepartitioned into 8 correlator tracking sets (see FIG. 4C) with eachtracking set containing an in-phase correlator, a quadrature-phasecorrelator, and an I early/late (IEL) correlator (or a Q early/late(QEL) correlator). In this case, the 24 correlators can lock 8satellites in parallel.

Of importance, in addition to the GPS data (which is provided by theWLAN), 4 pseudo-range measurements from 4 GPS satellites are needed tocompute a coordinate position. Specifically, because the GPS receivermust solve an equation including four variables x, y, z, and Δt, thepseudo-range measurements from four GPS satellites is needed.Advantageously, after the time and frequency offsets based on the firstsatellite are acquired, the time and the frequency offsets of the otherthree satellites can be derived from the first satellite.

Specifically, the time offset can be quickly computed for the othersatellites because the GPS receiver already has decoded GPS data and anapproximate location from the WLAN. For example, with one C/A code chipequal to 300 meters (1 millisecond/1023 chips), a search over 10kilometers will take approximately 30 milliseconds with one correlator.

The frequency offset includes two parts: the frequency offset of the GPSreceiver oscillator and the Doppler effect of satellite movement.Because the first satellite was selected to be the satellite having thehighest elevation (i.e. the satellite is substantially overhead), theDoppler effect is approximately zero (explained below). Thus, thefrequency offset is the difference between the GPS satellite oscillatorfrequency, which is very stable at 1.575 GHz, and the GPS receiverfrequency. Using the time and frequency offsets as well as the decodedGPS data, the GPS receiver can efficiently compute its relative positionwith respect to the GPS satellite (i.e. the speed of light×Δt=relativeposition R).

Because the frequency offset of the GPS receiver is now known, only theDoppler effect for each of the other three GPS satellites needs to becomputed. These Doppler effects can be quickly computed based on thedecoded GPS data.

With respect to the GPS satellite Doppler effect, note that the radiusof the Earth is about 6,378 kilometers and the GPS orbit is 20,200kilometers above the ground. Thus, with a period of 12 hours, the speedv of each GPS satellite is:(6378+20200)*10e3*2π/(12*60*60)=3865 meters/second.

The Doppler effect f_d can be computed using the following equation:f_d=f_c*v*cos θ/c

where f_c is the carrier (GPS satellite oscillator) frequency and c isspeed of light.

Thus, referring to FIG. 5, the satellite having the highest elevation,i.e. satellite 501, has an angle θ₁ of 90 degrees (wherein the cos θ₁ iszero). Therefore, the

Doppler effect would also be zero. However, the Doppler effect forsatellite 504 is:f _(—) d=f _(—) c*v*cos θ₂ /c=

where θ₂ is equal to (θ_e1+φ) and θ_e1 is the elevation angle. Thus, theworst case Doppler effect for satellite 504 would be the case where 0 is180 degrees (wherein the cos θ₂ is one). In this case, f_d=1.575e9*3865/3e8=20 kHz.

System Overview Including WLAN and WLAN Aided GPS

Advantageously, the position techniques using WLAN and the WLAN aidedGPS are complementary. Specifically, if the client is operating within aWLAN, then a client can quickly compute its relative and/or a coordinateposition using information from an access point. At this point, no GPStracking is necessary because an acceptably accurate location of theclient can be provided solely via the WLAN.

When the client disassociates with the WLAN, the WLAN can provide theclient with decoded GPS data, thereby ensuring quick and reliablepositioning capability. Specifically, the information provided by theWLAN can be used to determine which satellite has the highest elevationangle, i.e. the satellite that is directly overhead. Therefore, the GPSreceiver in the client has the greatest probability of not being blockedwhen searching for this satellite. Additionally, once the client islocked to this satellite, the time and frequency offsets of othersatellites can be quickly computed.

As shown in FIG. 6, the components associated with a radio frequency(RF) front end can be advantageously shared between WLAN and GPS becausethey are not running at the same time. Specifically, WLAN baseband 610and GPS baseband 611 can share an antenna 600, a low noise amplifier(LNA) 601, mixers 602/607, a filter 603, a frequency synthesizer 604, again stage 605, and an analog to digital (A/D) converter 609. Note thatFIG. 6 illustrates only some of the components of a WLAN system. Othercomponents needed or desired for operation of WLAN baseband 610 couldalso be included in this system without affecting GPS baseband 611.

Of importance, WLAN baseband 610 and GPS baseband 611 operate atdifferent carrier frequencies. For example, WLAN baseband 610 canoperate at 5 GHz (IEEE 802.11a standard) or at 2.4 GHz (IEEE 802.11b)whereas GPS baseband 611 can operate at 1.575 GHz. In accordance withone feature of the invention, frequency synthesizer 604 can beprogrammed using a reference clock 606 to provide the appropriatefrequency to mixer 607, i.e. to effectively switch between WLAN baseband610 and GPS baseband 611.

Note that the additional circuitry needed for GPS baseband operation ina WLAN system is incremental. Specifically, the number of digitalcircuits for baseband GPS processing is estimated to be less than 50Kgates for 24 correlators. Thus, the cost for a 0.13 micron CMOS chip iscommercially negligible.

Exemplary Applications

The WLAN position technique can be used to provide E911 service for cellphones. As noted previously, conventional GPS receivers cannot workindoors or under other circumstances where a GPS signal cannot bereceived. However, a WLAN can advantageously provide accurate positioninformation to its clients using an access point, which already knowsits GPS location. Even when a client disassociates with the accesspoint, the WLAN can provide time, location, and decoded GPS data to theclient before disassociation. In this manner, the client cansignificantly reduce the time to acquire the necessary GPS satellitedata (i.e. on the order of seconds instead of minutes).

The WLAN position technique can advantageously augment the navigationtools of any type of wireless device, e.g. PDAs, cell phones, andnotebooks. For example, the wireless device can provide local maps andpoints of interest for later GPS navigation with WLAN aided information.These navigation tools could be used from any location, e.g. home, work,airports, restaurants, hotels, etc.

The WLAN position technique can advantageously be used in conjunctionwith location based applications. Specifically, a client can download alocation based application and its related database from the WLAN andthen move to a new location with or without WLAN connections. Forexample, a client could connect to an access point in a coffee shop inTokyo. At this point, an application server could quickly determine theclient's location through the access point and then feed local mapsalong with other local point of interests (POI) to the client throughthe access point. Using this information, the client could then use theWLAN position technique or the WLAN aided GPS technique to navigate to anew location.

Note that generally the positional accuracy provided by the WLAN deviceis proportional to the typical range of the radio propagation. Forexample, if a current standard has a radio propagation range of Nmeters, then the accuracy of positioning with the WLAN device using thatstandard is within N meters.

It will be understood that the disclosed embodiments regarding WLAN andWLAN aided GPS positioning techniques are exemplary and not limiting.Accordingly, is intended that the scope of the invention be defined bythe following Claims and their equivalents.

The invention claimed is:
 1. A method comprising: determining aplurality of access points in a communications system; determining alocation of a wireless device in the communications system based atleast in part on information from the plurality of access points, theinformation including received signal strength indicators; and acquiringa satellite positioning system signal at the wireless device based atleast in part on the location.
 2. The method of claim 1, wherein saiddetermining the location includes using a server of the communicationssystem to assist in at least part of said determining the location. 3.The method of claim 1, wherein said determining the location isperformed solely by the wireless device.
 4. The method of claim 1,further including using the satellite positioning system signal togenerate a modified location of the wireless device.
 5. The method ofclaim 1, further including storing the satellite positioning systemsignal for use by the wireless device.
 6. The method of claim 1, furtherincluding periodically acquiring the satellite positioning system signalfor use by the wireless device.
 7. A wireless device for operating witha communications system, the wireless device to perform a locationtechnique comprising: determining a plurality of access points in thecommunications system; determining a location of the wireless devicebased at least in part on information from the plurality of accesspoints, the information including received signal strength indicators;and acquiring a satellite positioning system signal at the wirelessdevice based at least in part on the location.
 8. The wireless device ofclaim 7, wherein said determining the location includes using a serverof the communications system to assist in at least part of saiddetermining the location.
 9. The wireless device of claim 7, whereinsaid determining the location is performed solely by the wirelessdevice.
 10. The wireless device of claim 7, further including using thesatellite positioning system signal to generate a modified location ofthe wireless device.
 11. The wireless device of claim 7, furtherincluding storing the satellite positioning system signal for use by thewireless device.
 12. The wireless device of claim 7, further includingperiodically acquiring the satellite positioning system signal for useby the wireless device.
 13. A wireless device comprising: means fordetermining a plurality of access points in a communications system;means for determining a location of the wireless device based at leastin part on information from the plurality of access points, theinformation including received signal strength indicators; and means foracquiring a satellite positioning system signal at the wireless devicebased at least in part on the location.
 14. The wireless device of claim13, wherein said means for determining the location includes a server ofthe communications system to assist in at least part of determining thelocation.
 15. The wireless device of claim 13, wherein said means fordetermining the location is included in the wireless device.
 16. Thewireless device of claim 13, further including means for using thesatellite positioning system signal to generate a modified location ofthe wireless device.
 17. The wireless device of claim 13, furtherincluding means for storing the satellite positioning system signal foruse by the wireless device.
 18. The wireless device of claim 13, furtherincluding means for periodically acquiring the satellite positioningsystem signal for use by the wireless device.
 19. A communicationssystem including a plurality of access points, a wireless device, and aserver, the wireless device and the server to perform a techniquecomprising: determining a plurality of access points using the wirelessdevice; determining a location of the wireless device using the serverbased on information from the plurality of access points, theinformation including received signal strength indicators of at leastsome of the plurality of access points; and acquiring a satellitepositioning system signal at the wireless device based at least in parton the location.
 20. The communications system of claim 19, furtherincluding using the satellite positioning system signal to generate amodified location of the wireless device.
 21. The communications systemof claim 19, further including storing the satellite positioning systemsignal for use by the wireless device.
 22. The communications system ofclaim 19, further including periodically acquiring the satellitepositioning system signal for use by the wireless device.